Rodent Horns: A Case of Natural or Sexual Selection?

Mylagaulidae is an extremely unique family of rodents that lived during the Miocene period. Thought to have developed from Aplodontinae, a family that includes the mountain beaver (Shotwell 1958), Mylagaulid fossils have been found in almost all of the North American faunas during this time period (Korth 1999). Mylagaulids, which are now extinct, are the only rodents known to have had horns. To better understand these extinct rodents, researchers must determine why no horned rodents exist today. Evidence containing fossil remains must be analyzed to determine the functions of the horns, the possible motives for their evolution, and the reasons for the animals’ extinction.

By R. Bruce Horsfall [Public domain], via Wikimedia Commons

By R. Bruce Horsfall [Public domain], via Wikimedia Commons

One important question yet to be answered among researchers is the exact origin of these rodent horns. Natural selection, the direct result from outside environmental pressures to survive, could have caused the evolution of horns. They could have also initially increased the success of mating for the males, making the horns the result of sexual selection (Arnold 1994).

If horns were exclusively the result of natural selection, the rodents should have exhibited several specific behaviors. First, these rodents would have needed the horns to survive everyday environmental pressures. Mylagaulids were large rodents that show some definite adaptations to their habitat (Hopkins 2005). For example, these rodents could have lived primarily above ground for a time until they were pressured from predators to find another home. While most rodents today use their claws and teeth to help with digging (Hopkins 2005), horns may have made mylagaulids more efficient burrowers. Therefore, horns could have helped select mylagaulids survive by digging underground tunnels to escape the threat of predators. Mylagaulids had characteristics such as large neck muscles and thickened nasal bones to indicate that they did use their heads for digging (Hopkins 2005), and some research has shown that the horns were thick and flat (Hopkins 2005). Other research shows that the skulls of these rodents were low and broad, characteristics not ideal for head-lift digging (Korth 1994). This does not necessarily mean that the rodents did not use their horns for digging, but it does indicate that the horns may have evolved for some other purpose.

A second behavior that would prove natural selection would be the mylagaulids’ use of the horns in defending against predators. The mylagaulids probably became extinct because of competition from species of the gopher family (Baskin 1980). It was this competition along with the changing landscape of the Miocene period that eventually drove the mylagaulids to live underground (Baskin 1980).  The horns, which were broad enough at the base to cover the neck and eyes and seemed to be adapted for defense (Hopkins 2005), may have evolved as mechanisms to help the rodents fend off predators. An example this type of evolution can be seen in the Bovidae family, which includes sheep, cattle, antelope, and bison. Interspecific competition led the horns of these animals to be shaped differently because of various fighting techniques (Lundrigan 1996). While arguments can be made for horn development due to environmental pressure, it is also possible that the horns evolved earlier because of sexual selective pressure.

There are also several behaviors that mylagaulids should have shown if the horns were a direct result of sexual selection. If the horns were sexually dimorphic, male-male combat would have been a common behavior in these types of rodents. The males may have used these horns to fight off rival males who were interested in stealing mates. Horn size in sheep, antelope, cattle, and bison plays a major role in sexual interactions; the larger the horns are in these males, the more successful they are at mating (Picard et al. 1994). Horns may have served the same purpose in mylagaulids. The males with the largest horns would have had an advantage during battles, and so they would have won the right to mate with the more fertile females. The males with the smallest horns or no horns at all would have had a lesser chance to mate; therefore, they were not able to pass their genes to the next generation.

A similar behavior supporting sexual selection would be the use of the horns for some type of guarding after mating. Paternity is extremely important to males of most species, who instinctively want their genetic traits to continue (Birkhead 40). The horns may have given the males a way of better ensuring paternity by allowing them to guard the females before and/or after copulation. One way might have been to defensively guard the entrance of the tunnels so that rival males could not enter and copulate with the female. For example, male golden hamsters sleep between the entrance of the burrow and the female to fend off any male competition (Birkhead 179). When they do decide to copulate with the female, they do so frequently to increase their chances of fathering the offspring (Birkhead 179). Another example is that of the male purple martin, who also fends off rival males until he ensures paternity. Once the female begins to incubate the eggs, he will allow other males into the nest. He then will show promiscuous behavior by leaving to copulate with unattended females (Birkhead 39). A male mylagualid could have done the same, ensured paternity with his mate and then traveled to other underground tunnels to mate with other females and father their offspring as well. Pre- or post-copulatory guarding would have allowed a male the security of protecting his sperm and fathering multiple offspring.

Finally, if horns were the result of sexual selection, mylagaulid males would have used the horns as a form of sexual display to attract females as potential mates. In many species, males tend to be larger than females for this very reason (Judson 67). An example of such sexual display can be seen in male sea lions, which flaunt a large mane, grow up to three times larger than the female sea lions, and can attract many females at one time to become part of their harem (Judson 81). Another example is clearly observed in male peacocks, which fan their radiant tail feathers in hopes of inviting a suitable mating partner (Judson 82). Similarly, males of the Bovidae family have more success mating if they posses large horns (Picard et al. 1994). Males of these all species use their appearance to intimidate rival males as well as attract the most selective females. Females want their offspring to be fit and healthy; therefore, they try to choose the best-looking males as mates. The horns of mylagaulids could have developed because they were a trait females found desirable and therefore wanted their offspring to posses.

By F. John [Public domain], via Wikimedia Commons

By F. John [Public domain], via Wikimedia Commons

Sexual selection would not only have given mylagaulid males more success at mating and ensuring paternity, but it would also have allowed females to determine which sperm fertilizes their eggs. There was most likely an evolutionary “arms” race between males and females. Mylagaulid females may have evolved ways of avoiding or ridding themselves of sperm deemed unfit from certain males. For example, females in other species exercise this form of sexual selection by expelling the unwanted sperm once it has entered their bodies. This ensures that a particular weak male will not have the chance to father any offspring and pass on his undesirable traits.

Additionally, female mylagaulids would probably have found other ways to encourage sperm competition. One such behavior may have been engaging in more promiscuous mating habits. The more males a female can find to mate with increases her chances of birthing healthy offspring, so females look for suitable times to escape and mate with other males. The perfect opportunity may have presented itself as males were fighting each other in competition for food or mating rights. This situation would have allowed “sneaks,” or other males that may not otherwise have a chance, to mate with the more dominate males’ females.

This type of promiscuous behavior benefits the female in several ways. First, the female would be more likely to have all of her eggs fertilized (Birkhead 201). One male mylagaulid may not have healthy enough sperm to successfully fertilize his mate’s egg, and if the primary male is sterile, the female will never get pregnant. This is problematic for obvious reasons, mostly because the species cannot reproduce and will travel toward extinction. An example of this can be observed in red-winged blackbirds. Males of this species have large harems, which could lead to a sperm shortage (Birkhead 205). These males transfer about twelve million sperm at one time (Birkhead 205), which could potentially leave some females unfertilized.

Another benefit to promiscuous behavior would have been the chance for mylagaulid females to ensure that their offspring have sufficient care after birth (Birkhead 215). Females of some species trade sex for parental care. If a mylagaulid male was willing to stay and take care of the offspring, the offspring had a better chance of surviving. In response to these promiscuous female behaviors, males would have found ways of guarding the female or keeping their own sperm in the female more efficiently. This evolutionary “arms” race would have been a very important part of the survival of mylagaulids.

As with any adaptation, trade-offs are bound to occur between sexual selection and natural selection. Mylagaulid horns surely evolved for some beneficial reason to the animals’ survival; however, as with other morphological features, the horns probably developed at a high cost and might have been detrimental in other ways. Horns in other species have developed with such trade-offs. For example, horned beetles use their horns to block entrances to underground tunnels which contain females. The horns also keep rival males out of the tunnels (Emlen 2001). Although this behavior is an advantage to the reproductive fitness of horned beetles, other morphological structures of the species suffer. Normally, the horns are located on the head, which results in smaller eyes and obstructed vision (Emlen 2001). Researchers also found that the horns sometimes develop near the wings, which causes the wings to develop incorrectly or incompletely (Emlen 2001).

Another example of such a trade-off exists in the Bovidae family, whose horns have proven to be a source of heat loss (Picard et al. 1994). Even though the horns increase the rate of successful mating and combative survival in these animals, the heat loss seems to counterbalance this advantage. If this loss of body heat disrupts the physiology of the organisms during winter, then the animals with the largest horns could possibly die during the winter months or in other cold temperatures (Picard et al. 1994).

The horns of mylagaulids could very well have developed with similar fitness trade-offs. The male mylagaulids did have small eyes, a trait the horns could have caused (Hopkins 2005), and poor eyesight could have led to their extinction for a number of reasons. The mylagaulids may not have been able to see, and therefore avoid, oncoming predators. Also, female mylagaulids could have had trouble discerning “fit” males from undesirable males, which might have led them to select inferior mates. The horns may have quickly turned from advantageous to disastrous.

By James Gidley ( [Public domain], via Wikimedia Commons

By James Gidley ( [Public domain], via Wikimedia Commons

Research shows that the Mylagaulidae family developed horns for a definite reason or purpose, whether as a result of natural selection or sexual selection. Most likely, the horns increased both the survival rate and reproductive success of this family. The horns probably also began an evolutionary race between the sexes. By studying the different evolutionary adaptations and behaviors of the Mylagaulidae family and other past species, researchers can gain insight into the development and interactions of modern living organisms.

Arnold, S. (1994). Is there a unifying concept of sexual selection that applies to both plants and animals? The American Naturalist, 144:1-12.

Baskin, J. (1980). Evolutionary reversal in Mylagaulus ( Mammalia, Rodentia) from the late Miocene of Florida. American Midland Naturalist, 104:155-162.

Birkhead, T. (2000). Promiscuity: An Evolutionary History of Sperm Competition. Cambridge, MA: Harvard University Press.

Cook, H. and Gregory, J. (1941). Mesogaulus praecursor, a new rodent from the Miocene of Nebraska. Journal of Paleontology, 15:549-552.

Emlen, D. (2001). Costs and the diversification of exaggerated animal structures. Science, 291:1534-1536.

Hopkins, S. (2005). The evolution of fossoriality and the adaptive role of horns in the Mylagaulidae (Mammalia: Rodentia). Proceedings of the Royal B, 272:1705-1713.

Judson, O. (2002). Dr. Tatiana’s Sex Advice to all Creation. New York: Henry Holt and Company, LLC.

Korth, W. (1999). Hesperogaulus, a new genus of Mylagaulid Rodent (Mammalia) from the Miocene (Barstovian to Hemphillian) of the Great Basin. Journal of Paleontology, 73:945-951.

Korth, W. (1994). The Tertiary Record of Rodents in North America. New York: Plenum Press.

Lundrigan, B. (1996).Morphology of horns and fighting behavior in the family Bovidae. Journal of Mammalogy, 77:462-475.

Picard, K., Thomas, D., Festa-Bianchet, M., and Lanthier, C. (1994). Bovid horns: An important site for heat loss during winter? Journal of Mammalogy, 75:710-713.

Shotwell, J. (1958). Evolution and biogeography of Aplodontid and Mylagaulid rodents. Evolution, 12:451-484.




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